Enhance support for MySQL XA connections

As generally accepted, the MySQL support for XA is somewhat broken.

If you do the following:

• begin a transaction
• get an XAConnection from MySQL,
• get a connection from the XAC
• do some operations,
• close the connection
• get another connection from XAC
• try to do some operations -> MYSQL throws an exception stating that it does not support a XA START RESUME

But, this leads to certain undesirable behavior inside the JBOSS JTA codebase. It is resulting in not calling XAC.close(). We are using a custom pool for limiting the number of XAConnections we create - which gets maxed out because of this. Am attaching a patch for ConnectionImple that fixes the logic in the close() method. Also has a couple of jdk6 methods that allow the client to access the underlying native conneciton..

Am also adding a patch for the TransactionImple - which ensures that we call XAResource.rollback() - cleaning up the XAC for subsequent reuse.